On October 19, 2012, about 1412 eastern daylight time, a Cessna 560XL, N576QS, received minor damage during gear retraction after takeoff at Nantucket, Massachusetts (ACK). The airline transport pilot, co-pilot, and one passenger were not injured. The airplane was registered to a corporation and was operated by NetJets Aviation, Inc. under the provisions of 14 Code of Federal Regulations Part 135 as an air taxi flight. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed. The flight originated at ACK and was destined for Lexington, Kentucky (LEX). The flight crew reported that, at 400 feet above ground level after takeoff, the flaps were retracted. Shortly thereafter, tower personnel informed the crew that fuel was pouring from the left wing. The crew also noted hydraulic and stabilizer annunciator lights and a gear unlocked indication. A slight rumble could be felt in the airframe. The left wing fuel indicator decreased approximately 500 pounds and the crew noted heaviness in the right wing. The flight remained in the traffic pattern and returned to ACK for an uneventful landing with a fuel imbalance of about 1,000 pounds. An inspector with the Federal Aviation Administration responded to the accident site and examined the airplane. The left main gear aft trunnion pivot pin was not in place, resulting in separation of the aft portion of the left main gear trunnion from the aft trunnion fitting. The end of the aft trunnion punctured the interior of the gear well, resulting in minor damage to surrounding structure. The pin was later found loose and undamaged in the flap track area of the left wing. The aft trunnion pivot pin retaining bolt that was intended to restrain the pivot pin was in place and secured with a nut and cotter pin. A review of maintenance records for the airplane revealed that the left main gear trunnion was removed and replaced with a new unit on September 1, 2012 by a maintenance vendor. According to the Cessna 560XL Maintenance Manual, when installing the aft main landing gear trunnion, the trunnion pivot pin must be slid forward, aligning a hole in the trunnion with a hole in the pivot pin. The trunnion pivot pin is then secured in place with the retaining bolt, washers, nut, and cotter pin.